This document discusses metaprogramming in Python using metaclasses and decorators. It begins with an introduction to why metaprogramming is useful and powerful. It then covers decorators in depth, explaining how they work and providing examples. Finally, it discusses metaclasses, explaining how they allow modifying class creation and providing examples like logging method calls and validating class initialization.

1. metaprogramação
em python:
metaclass e decorators

2. @felipevolpone
github.com/felipevolpone
felipevolpone.com

3. agenda
por que aprender metaprogramação?
decorators
metaclasses
usando o inspect
perguntas

4. por que
metaprogramação?
aprender

6. por que
é poderoso
conhecimento
reutilizar código
aumentar abstração
criar APIs mais ricas

7. decorators
https://www.python.org/dev/peps/pep-0318/
python 2.4

8. o que é
!
!
@staticmethod
def say_hello():
return "Hello Python Brasil :)"
!
!
@app.route('/')
def hello_rest():
return "Hello folks :)"

9. funções são objetos
de primeira classe

10. como funciona
def do_nothing(func):
def internal():
return func()
return internal
!
@do_nothing
def foo():
return "bar"
!
print foo()
> "bar"

11. #decorator
def example(func):
…
!
#@example
def foo():
return "bar"
!
foo = example(foo)
!
print foo()
> "bar"

12. #@example
def foo():
return "bar"
!
foo = example(foo)

13. def example(func):
def internal():
return func()
return internal
!
#@example
def foo():
return "bar"
!
foo = example(foo)

14. def example(func):
def internal():
return func()
return internal
!
#@example
def foo():
return "bar"
!
foo = example(foo)

15. def example(func):
def internal():
return func()
return internal
!
#@example
def foo():
return "bar"
!
foo = example(foo)

16. def example(func):
def internal():
return func()
return internal
!
#@example
def foo():
return "bar"
!
foo = log(foo)

17. def example(func):
def internal():
return func()
return internal
!
#@example
def foo():
return "bar"
!
foo = example(foo)
}

18. print foo
<function foo at 0x105909c80>
!
foo = example(foo)
print foo
<function internal at 0x105909cf8>

19. def example(func):
def internal():
return func()
return internal
!
#@example
def foo():
return "bar"
!
foo = example(foo)
print foo()

20. def example(func):
def internal():
return func()
return internal
!
def foo():
return "bar"
!
foo = example(foo)
print foo()
> "Hello"

21. def example(func):
def internal():
return func()
return internal
!
def foo():
return "bar"
!
foo = example(foo)
print foo()
> "Hello"

22. def example(func):
def internal():
return func()
return internal
!
@example
def foo():
return "bar"
!
print foo()
> "Hello"

23. from datetime import datetime
!
def log(func):
def internal():
now = datetime.now()
.strftime(“%d/%m/%Y %H:%M:%S")
print "Funcao chamada: %s as %s" %
(func.func_name, now)
return func()
return internal
!
print foo()
> "Funcao chamada: foo as 5/11/2015
16:45:36"
> "bar"

24. def log(func):
def internal():
now = …
print "Funcao chamada ..."
return func()
return internal
!
@log
def say(message):
return message
@log
def goodbye():
return “goodbye"
!
print goodbye()
> Funcao chamada: goodbye as 5/11/2015 16:30:23
> goodbye
print say("Python Brasil")
> TypeError: internal() takes no arguments (1 given)

25. !
def log(func):
def internal(*args, **kwargs):
now = …
print "Funcao chamada: …"
return func(*args, **kwargs)
return internal

26. args é iterável
kwargs é {}

27. def example(func):
def internal(*args, **kwargs):
return func(*args, **kwargs)
return internal
!
@example
def sum_base(a, b, base=12):
return a+b+base
!
print sum_base(6, 10, base=12)
> 28

28. def example(func):
def internal(*args, **kwargs):
print args, type(args)
print kwargs, type(kwargs)
return func(*args, **kwargs)
return internal
!
@example
def sum_base(a, b, base=12):
return a+b+base
!
print sum_base(6, 10, base=12)
> (6, 10) <type 'tuple'>
> {'base': 12} <type 'dict'>
> 28

29. combinando
@decorators

30. def timestamp(func):
def internal(*args, **kwargs):
print "funcao chamada em %s" % (datetime.now())
return func(*args, **kwargs)
return internal
!
def log(func):
def internal(*args, **kwargs):
print func
return func(*args, **kwargs)
return internal
!
@timestamp
@log
def somar(a,b,base=12):
return a+b+base
!
print somar(6, 7, base=12)
> funcao chamada em 2015-07-17 17:32:48.209305
> <function somar at 0x100872cf8>
> 25

31. @timestamp
@log
def sum_base(a,b,base=12):
return a+b+base
!
sum_base = timestamp(log(sum_base))

32. !
@app.route('/about')
def about():
return 'The about page'
!
!
@unittest.skip('Failing test')
def test_example(self):
self.assertEquals(1, 2)
!

33. def authentication(msg):
def decorator(func):
def internal(*args, **kwargs):
user = get_session_user()
if user.valid:
return func(*args, **kwargs)
else:
raise Exception(msg)
return internal
!
return decorator

34. @authentication("User is not valid")
def say_hello():
return "Hello"
!
say_hello =
authentication(“User is not valid")(say_hello)
!
print say_hello()
> "Hello”
!
#quando usuario nao eh valido
print say_hello()
> raise Exception(msg)
Exception: User is not valid

35. @json
@flask.get('/products/')
def products():
return {'products': get_products()}
!
@html
def get_static_file(file_path):
return open(file_path)
!
@auth('admin')
def delete_user():
...
!
@convert_object
@app.post('/user')
def get_user(model_user)
# transforma json em objeto python
model_user.put()

36. cobre 90%
dos casos

37. def example(func):
def internal(*args, **kwargs):
"""Useless decorator"""
return func(*args, **kwargs)
return internal
!
@example
def say_hello():
"""A simple function that says hello"""
return "Hello"
!
print say_hello.func_name
print say_hello.func_doc
> "internal"
> "Useless decorator"

38. def example(func):
def internal(*args, **kwargs):
"Useless decorator"
return func(*args, **kwargs)
internal.func_name = func.func_name
internal.func_doc = func.func_doc
return internal
!
@example
def say_hello():
"Function that says hello"
return "Hello"
!
print say_hello.func_name, say_hello.func_doc
> “say_hello" "Function that says hello"

39. from functools import wraps
!
def example(func):
@wraps(func)
def internal(*args, **kwargs):
"Useless decorator"
return func(*args, **kwargs)
return internal
!
@example
def say_hello():
"Function that says hello"
return "Hello"
!
print say_hello.func_name, say_hello.func_doc
> “say_hello" "Function that says hello"

40. from functors import wraps
!
def example(func):
@wraps(func)
def internal(*args, **kwargs):
"Useless decorator"
return func(*args, **kwargs)
return internal
!
@example
def say_hello():
"Function that says hello"
return "Hello"
!
print say_hello.func_name, say_hello.func_doc
> “say_hello" "Function that says hello"
/usr/lib/python2.7/functools.py
!
WRAPPER_ASSIGNMENTS = ('__module__', '__name__', ‘__doc__')

41. import wrapt
http://pyvideo.org/video/2617/advanced-methods-for-creat

42. import wrapt
!
@wrapt.decorator
def example(func):
def internal(*args, **kwargs):
"Useless decorator"
return func(*args, **kwargs)
return internal

43. metaclasses

44. entendendo
classes

45. objetos de primeira classe
class User(object):
pass
!
print dir(User), User.__dict__
!
> ['__class__', '__doc__','__getattribute__',
'__hash__', '__init__', '__module__', '__new__',
'__repr__', '__setattr__', '__sizeof__', ‘__str__’]
!
> {'__dict__': <attribute '__dict__' of 'User' objects>,
'__module__': '__main__',
'__weakref__': <attribute '__weakref__' of 'User' objects>,
'__doc__': None}

46. @staticmethod
def hello():
print 'hello'
!
User.our_method = hello
User.our_method()
objetos de primeira classe

47. https://docs.python.org/2/library/functions.html#type

48. def init(instance, name):
instance.name = name
instance.track = 'web'
!
EventRuntime =
type('Event', (object,), {'__init__': init})
u = EventRuntime('PythonBrasil[11]')
!
print u.name
> “PythonBrasil[11]“
print u.track
> "web"
print type(u)
> <class '__main__.Event'>

49. 'pythonbrasil'.__class__
> str
!
type('pythonbrasil')
> str
!
isinstance('2015',str)
> True
a = User()
!
type(a)
> '__main__.User'

50. type(User)
> type
!
type(str)
> type
metaclass class
class
class
isinstance(User, type)
> True
!
isinstance(str, type)
> True

51. User type
metaclass class
instance
str
instance
type
user
instance
instance
"pybr11"
instance

52. __call__ __init__
__new__
instância
user = User()
metaclassclass
1
3 2
4
__call____init__
1.1
1.2

53. !
class Log(type):
pass
!
class Example(object):
__metaclass__ = Log
!
class Log(type):
pass
!
class Example(metaclass=Log):
pass
2.x
3.x

54. class MyMetaclass(type):
pass
!
class Person(object):
__metaclass__ = MyMetaclass
!
print type(Person)
> <class '__main__. MyMetaclass'>

55. exemplos

56. 1.

57. import inspect
!
def log(func):
@wraps(func)
def inner(*args, **kwargs):
print '[LOG] %s was called' % (func.func_name,)
return func(*args, **kwargs)
return inner
!
class Log(type):
!
def __init__(cls, *args, **kwargs):
for prop in args[2]:
method = getattr(cls, prop)
!
if inspect.ismethod(method):
decorated_method = log(method)
setattr(cls, prop, decorated_method)
!
return type.__init__(cls, *args, **kwargs)

58. class Person(object):
__metaclass__ = Log
!
def say(self):
return 'hi'
!
p = Person()
print p.say()
> "[LOG] say function called"
> "hi"

59. 2.

60. pessoa_json = {'name': 'felipe', 'age': 22}
class Pessoa(object):
def __init__(self, name, age):
nova_pessoa = Pessoa()
for key, value in pessoa_json.items():
setattr(nova_pessoa, key, value)

61. import inspect
!
!
class ModelProtocol(type):
!
def __new__(cls, name, parents, properties):
!
if '__init__' in properties:
init_function = properties['__init__']
init_inspection = inspect.getargspec(init_function)
!
if init_inspection.defaults is None and
len(init_inspection.args) > 1:
raise Exception(‘The object must have a keywords
arguments in __init__')
!
return type.__new__(cls, name, parents, properties)

62. import inspect
!
!
class ModelProtocol(type):
!
def __new__(cls, name, parents, dct):
!
if '__init__' in dct:
init_function = dct['__init__']
init_inspection = inspect.getargspec(init_function)
!
if init_inspection.defaults is None and
len(init_inspection.args) > 1:
raise Exception(‘The object must have a keywords
arguments in __init__')
!
return type.__new__(cls, name, parents, dct)
ArgSpec(args=['self', ‘name'], varargs=None, keywords=None, defaults=None)

63. class User(object):
__metaclass__ = ModelProtocol
!
def __init__(self, name, age):
pass
class User(object):
__metaclass__ = ModelProtocol
!
def __init__(self, name=None, age=18):
pass

64. class User(object):
__metaclass__ = ModelProtocol
!
def __init__(self, name, age):
pass
!
any_user = User('speaker', 22)
> Exception: A modelobject must have
a keywords arguments in __init__

65. class User(object):
__metaclass__ = ModelProtocol
!
def __init__(self, name=None, age=18):
...
!
user_json = {'name': 'felipe', 'age': 22}
new_user = User()
for key, value in user_json.items():
setattr(new_user, key, value)
!
print new_user.name, new_user.age
> "felipe 22"

66. !
user_json = {'name': 'felipe', 'age': 22}
!
def to_instance(clazz, object_json):
instance = clazz()
for prop, value in object_json.items():
if prop in instance.__dict__:
setattr(instance, prop, value)
return instance
!
new_user = to_instance(User, user_json)

67. descriptors
https://speakerdeck.com/ramalho/python-encapsulation-with-descriptors-1

69. from google.appengine.ext import ndb
!
class Event(ndb.Model):
name = ndb.StringProperty(required=True)
description = ndb.TextProperty(required=True)
logo = ndb.BlobKeyProperty(default=None)
date = ndb.DateTimeProperty()

70. @felipevolpone
felipevolpone.com